1. Field of the Invention
The present invention relates to a synchronous light detector for attaining a scanning timing of a light beam in an optical scanning apparatus equipped with light-deflecting means constituted by a rotary polygon mirror or the like.
2. Description of the Prior Art
Optical scanning apparatus, which are apparatus for optically scanning a surface to be scanned by deflecting a light beam, have been known as apparatus for writing optical information onto the surface and reading out information therefrom.
Conventionally known as such an optical scanning apparatus is one employing a system in which light-deflecting means deflects a light beam, thus deflected light beam is focused like a spot by an imaging lens onto a surface to be scanned, and the surface is optically scanned therewith. Such an optical scanning apparatus uses a rotary polygon mirror, for example, as the light-deflecting means.
The rotary polygon mirror is made of a right-regular prism whose side faces form a mirror surface. When the rotary polygon mirror is rotated while a light beam is incident on the mirror surface along a fixed direction, the light beam reflected by the mirror surface is periodically deflected. Thus deflected light beam is focused like a spot by the imaging lens onto the surface to be scanned. As the light beam is deflected, the spot of the light beam moves on the surface to be scanned, thereby optically scanning the latter.
Meanwhile, the above-mentioned conventional optical scanning apparatus is equipped with a synchronous light detector for attaining a timing for optical scanning. This synchronous light detector is configured such that the light beam deflected by the rotary polygon mirror toward its optical scanning area by way of the imaging lens is detected as synchronous light by a light-receiving device, so as to attain synchronization of a scanning start timing of the light beam.
Also, a cylindrical lens having a refracting power only in the direction orthogonal to the scanning plane of the light beam is disposed between the imaging lens and the light-receiving device, while the deflecting point of the light beam caused by the rotary polygon mirror and the imaging point on the light-receiving surface of the light-receiving device are made optically conjugate with each other within a plane orthogonal to the scanning direction of the light beam upon attaining synchronization of the scanning start timing of the light beam (as disclosed in Japanese Patent Publication No. 7-43462, for example).
However, when a point on the light-receiving surface of the light-receiving device is disposed at the position optically conjugate with the deflecting point of the light beam caused by the rotary polygon mirror, while the light beam forms an image on the light-receiving surface of the light-receiving device; if there is a flaw on the light-receiving surface of the light-receiving device or dirt is attached thereto, and the image is formed at the location where this flaw or dirt exists, then the light beam is scattered on the light-receiving surface, whereby synchronization signals may not be detected correctly. As a consequence, there has been a possibility that it becomes hard to attain synchronization of the scanning start timing of the light beam.